肥胖指的是體脂肪過度堆積而對健康造成負面影響的身體狀態[8]:6。肥胖最初的定義以身體質量指數(Body Mass Index, BMI)為指標,之後也加入了其他標準,例如評估腰臀比或其他心血管病症風險因子[22][23]:14。BMI與體脂肪率和體脂肪總量都密切相關[24]。兒童的健康體重因性別與年齡而有不同的標準,此標準需透過與同年齡性別的其他孩子(常模)比較來決定。舉例而言,兒童與青少年的肥胖定義為BMI高於95%的同儕(95百分位以上)[25]。用於決定兒童與青少年肥胖與否的常模資料來自1963年至1994年,因此並沒有反映近年平均體重的上升[26]。
肥胖症是最常見的可預防性致死因素(英語:Preventable causes of death)之一[43][44][45]。歐美的大規模研究指出身體質量指數(BMI)介於20到25公斤/平方公尺的非吸煙者[42][46]以及BMI介於24-27公斤/平方公尺的吸煙者在非吸煙和吸煙者群體中分別有最低的死亡風險,高於或低於這個範圍死亡風險都會上升[47][48]。亞洲人則在BMI達到22到25公斤/平方公尺時死亡率就開始上升[49]。BMI大於32公斤/平方公尺的女性未來十六年內的死亡率高達常人的兩倍[50]。肥胖每年帶走111,909至365,000條美國人的生命[2][45];歐洲有7.7%的死亡(相當於約100萬人)和過重相關[51][52]。平均而言,肥胖會減少6-7年的餘命[2][53],當一個人的BMI達到30-35公斤/平方公尺間時,餘命會減少2-4年[46],而嚴重肥胖者(BMI大於40公斤/平方公尺)則會減少十年餘命[46]。
不同的區域與國家的人均飲食熱量攝取(英語:Dietary energy supply)差別很大,並會隨着時代而有明顯改變。從1970年代早期到1990年代晚期,除了東歐地區外,全球的人均每日熱量攝取(購買的食物量)都在上升。1996年,人均每日熱量供應最高的國家是美國,高達3,654卡路里(15,290千焦耳),到了2003年更來到了3,754卡路里(15,710千焦耳)。1990年代晚期,歐洲人平均每人每日購買3,394卡路里(14,200千焦耳)的食物,而亞洲的發展中國家每人每日的食物購入量有2,648卡路里(11,080千焦耳),而撒哈拉以南非洲則是2,176卡路里(9,100千焦耳)[95]。總熱量的攝取量與肥胖的發生有關[96]。
即使現在的飲食指引(英語:List of nutrition guides)相當普及[97],但飲食過量與飲食內容不健康的問題卻未因此改善[98]。自1971年到2000年,美國的肥胖率從14.5%爬升到30.9%[99],而平均的熱量攝取也在同期上升。女性的每日熱量攝取從1971年的1,542卡路里(6,450千焦耳)到2004年的1,877卡路里(7,850千焦耳),平均上升了335卡路里(1,400千焦耳);而男性從1971年的2,450卡路里(10,300千焦耳)到2004年的2,618卡路里(10,950千焦耳),平均增加了168卡路里(700千焦耳)。飲食熱量的上升主要來自於碳水化合物攝取量的上升,而非油脂攝取的上升[100],多出來的碳水化合物主要來自含糖飲料和馬鈴薯片[101]。現代美國年輕人的飲食中,含糖飲料佔了每日熱量攝取的近四分之一[102];一般認為碳酸飲料、調味果汁、調味茶、運動及能量飲品等含糖飲料在肥胖率的上升中居功厥偉[103][104],並造成代謝症候群與第2型糖尿病的風險上升[105]。
肥胖的發生與維持有許多種可能的病理生理機轉[152],但這個研究領域一直乏人問津,在1994年時,弗萊德曼(J. M. Friedman)的研究團隊才發現瘦素基因[153]。弗萊德曼的研究團隊認為瘦素是影響飽足感的因子。實驗中發現胖胖鼠(英語:ob/ob mouse)(一種意外發現,具有過度食慾並且極端肥胖的實驗用小鼠)帶有突變瘦素基因,因此展現肥胖的表現型,這個結果開啟以瘦素來治療人類肥胖的可能性,但不久後卡羅(J. F. Caro)的研究團隊就發現無法在肥胖的人身上找到任何瘦素基因的突變;另一方面,在肥胖者身上的瘦素基因表現量上升,反而指出了肥胖者身上可能有瘦素抵抗現象[154]。在發現瘦素後,研究者陸續發現許多其他影響食慾、進食、脂肪儲存、胰島素抗性的激素,舉例來說飢餓素、胰島素、食慾素、雙酪胺酸肽3-36(英語:peptide YY)、膽囊收縮素、脂聯素等因子都是學者研究的對象。脂激素(英語:Adipokine)是由脂肪組織釋放出的訊息因子,它們的作用會影響許多肥胖相關的疾病。
自文藝復興時代開始,部分上層階級開始炫耀自己的體胖,在英王亨利八世與亞歷山卓·達·布羅(英語:Alessandro dal Borro)的畫像中就能看到這樣的趨勢[18]。知名畫家魯本斯經常描繪魯本斯式的女性全身像,雖然表現上也強調多產,但在他畫中的女性仍然維持沙漏狀的身形[210]。到了十九世紀,整個西方世界對於肥胖的看法開始改變,在肥胖作為財富與地位的象徵許多世紀以後,纖瘦取而代之成為新的追求目標[18]。
在許多國家,寵物肥胖(英語:Obesity in pets)是常見的問題。在美國,有23到41%的狗過重,當中有5.1%達到肥胖的程度;貓的肥胖率則稍高一些(6.4%)[238]。在澳洲,獸醫統計的犬隻肥胖率為7.6%[239]。犬隻肥胖的風險與飼主是否肥胖相關,但貓隻的風險則與飼主肥胖與否無關[240]。
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